Optical packet header identifier, optical router incorporating the same therein, and optical routing method using the router

ABSTRACT

An optical packet header identifier having a simplified configuration and being superior in reliability, stability, and economical efficiency, an optical router incorporating the identifier therein, and a routing method using the optical router are provided. The optical packet header identifier includes an optical waveguide, optical focusing elements, and a photo receiver. Tilted gratings for diffracting an incident optical beam and emitting the beams as diffracted optical beams to the outside of the waveguide are formed within the optical waveguide. The tilted gratings are not formed uniformly in a longitudinal direction of a core of the optical waveguide, but are arranged at intervals. The length of a portion containing a set of gratings and the length of a portion containing no gratings can be defined in increments of length “L”. “L” equals to the spatial length which 1 bit in an optical signal occupies.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical packet header identifier forverifying whether or not an optical address code added to an opticalpacket signal transmitted from outside coincides with an address codepreviously given to an optical packet receiver without photoelectricconversion while maintaining the added code in the form of light and foroutputting a result of the coincidence of codes as an electrical signaloutput in the event the two codes coincide with one another, an opticalrouter incorporating the identifier therein, and an optical routingmethod using the optical router.

2. Description of the Related Prior Art

Advent of the Internet community has been drastically increasingcommunication traffic. Therefore, there is a demand for realization of anetwork that is able to accept the increase in communication traffic,have a high-capacity, operate at higher rate, and have at lowercommunication cost. Optical fiber communication technology is very theone that meets such requirements.

When focusing attention on the field of communication protocol, aconnectionless communication typified by Internet Protocol (IP) isbecoming increasingly dominant over a circuit switching connectiontypified by a telephone network. To achieve a high-capacity andhigh-speed communication system of the type used for connectionlesscommunication, it is desirable to be able to perform optical signaltransmission throughout from a transmitting terminal to a receivingterminal without any photoelectric conversion.

Routing is a technique for selecting an optimal path to be used from aplurality of communication paths in order to transmit an IP packet to afinal destination. In an optical communication system, a wavelengthrouting system that employs a wavelength as address information todetermine the destination of signal light has been known as a routingtechnique that uses an optical signal as it is. However, this routingtechnique can be applied only to a high-speed/capacity portion ofcommunication network because a wavelength resource, i. e., the numberof wavelengths to be allocated to individual addresses is limited.Currently, it is difficult to deliver an IP packet to an access paththat needs a number of addresses while maintaining the packet in theform of light.

A technique using an optically encoding/decoding device for encoding alight signal, and in turn, decoding the encoded light signal isdisclosed in Japanese Patent Application Laid-open No. 2001-177565.

FIG. 1 is the exemplary configuration of an optical encoder (note thatthe optical encoder can be used not only for encoding a light signal butfor decoding the same) composed of 8-tip optical bipolar encoder of thetype used in PLC (Planar Lightwave Circuit) in the above-describedpatent. A light pulse input to the optical encoder is branched intoeight tip pulses that are made to have a time delay of 5 ps betweenadjacent pulses and equal intensity by operating a tunable optical tap41 and an optical delay line 42. Each of the branched tip pulses isprocessed such that a phase shift “0” or “π” due to a thermal-opticaleffect by an optical phase shifter 43 is given to the optical phase ofthe tip pulse ,and then encoded by again combining the tip pulsestogether through a combiner 44. The given combination of phase shiftsthus corresponds to one code. Each of the optical phase shifters 43 iscontrolled in response to an address code, thereby producing a desiredoptical bipolar code. In turn, when the optical bipolar code is input tothe same optical encoder, a correlation between the optical code inputthereto and the combination of phase shifts of the optical phase shifteris detected. A correlation signal having high intensity is output onlywhen the optical code input thereto and the combination of phase shiftsof the optical phase shifter coincide with one another, whereby the codeis identified.

In the above-described optical encoder employed in the disclosedtechnique, since encoding or decoding is performed by giving a phaseshift “0” or “π” to an electric field of light, the encoding or decodingis so sensitive to change in optical frequency. Furthermore, the opticalencoder is not practical for use because it has no compatibility withthe current optical fiber communication system in which data or addressis transmitted/received by modulating the intensity of light.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-describedproblems, and an object of the present invention is therefore toprovide: an optical packet header identifier, without calculation ofcorrelation in an electronic circuit, having a low power consumption,operating at higher rate, having a simplified configuration, and havinga superiority in reliability, stability and economical efficiency; anoptical router incorporating the identifier therein; and a routingmethod using the router.

An optical packet header identifier according to the first aspect of thepresent invention comprises: an optical waveguide, a tilted grating fordiffracting an optical beam propagating within a core of the opticalwaveguide toward the outside of the optical waveguide; and a set oftilted gratings constituted by the tilted grating and having a thicknessof gratings approximately equal to the length occupied in a directionalong an optical axis within the optical waveguide by 1-bit in a signalof an optical packet, the plurality of sets of tilted gratings beingencoded and arranged in a direction along an optical axis based on aspecific header code of the optical packet; optical beam focusing meansfor spatially focusing optical beams diffracted by all of the sets oftilted gratings; a first photo receiver for receiving the focusedoptical beams; and a second photo receiver for receiving an optical beampropagating though the optical waveguide at an output end of the opticalwaveguide.

The identifier is further constructed such that the optical waveguide isany one of an optical fiber and a channel waveguide formed on a planarsubstrate.

The identifier is further constructed such that the optical beamfocusing means is a slab waveguide provided to guide the diffractedoptical beams and having a parabolic reflecting end face for reflectingthe guided optical beams, and the first photo receiver is provided at afocal point of the slab waveguide.

The identifier is further constructed such that the optical beamfocusing means comprises a second optical fiber for diffracting thediffracted optical beams to make the optical beams travel in claddingmode by using tilted gratings formed in a core portion of the secondoptical fiber, and the first photo receiver is provided at an end faceof the second optical fiber to receive the optical beams traveling incladding mode within the second optical fiber.

The identifier is further constructed such that the optical beamfocusing means further comprises a third optical fiber which is locatedbetween the optical waveguide and the second optical fiber, and focusesthe diffracted optical beams from the optical waveguide onto a center ofthe second optical fiber.

The identifier is further constructed such that the optical waveguide isprovided on a front side of the third optical fiber and positioned aparta distance two times a focal length of the third optical fiber forfocusing, from the third optical fiber, and the second optical fiber isprovided on a rear side of the third optical fiber and positioned aparta distance two times a focal length of the third optical fiber from thethird optical fiber.

The identifier further comprises a fourth optical fiber in addition tothe third optical fiber, in which a center of the optical waveguide ispositioned at a front focal point of the third optical fiber and acenter of the second optical fiber is positioned at a rear focal pointof the fourth optical fiber, and the third and fourth optical fibersfocus diffracted optical beams from the optical waveguide onto thesecond optical fiber.

The identifier is further constructed such that the optical beamfocusing means is a “fθ” lens provided so that a central axis of thecore of the optical waveguide and a light receiving face of the firstphoto receiver have relationship represented by “fθ”, and a tilt angleof each of the plurality of sets of tilted gratings is determined sothat a diffraction direction associated with diffracted optical beamsfrom each of the plurality of sets of tilted gratings satisfiesrelationship represented by “fθ”.

The identifier is further constructed such that the optical waveguide isprovided to form a circle in a plane, the plane having the tiltedgrating tilted therein, and the optical beam focusing means comprises:an optical fiber acting as a cylindrical lens and provided so that theoptical fiber forms another circle around the same center as that of thecircle in the same plane as that for the optical waveguide to form thecircle, and a center of the core of the optical waveguide is positionedat a front focal point of the optical fiber; and a lens for focusingdiffracted images, transmitted from the optical fiber and formed in thecenter of the circle, onto the first photo receiver.

The identifier is further constructed such that light-diffractingefficiency of each of the plurality of sets of tilted gratings isdetermined so that intensity of diffracted optical beams from all of theplurality of sets of tilted gratings becomes uniform.

The optical packet header identifier according to the first aspect ofthe present invention is further characterized in that the opticalwaveguide is a semiconductor optical waveguide, the tilted grating has aspatial periodic refractive index variation generated in the core by aband-filling effect, the effect being caused when carriers are injectedinto the semiconductor optical waveguide, and the set of tilted gratingsis formed by conducting current between grid-shaped electrodes which isprovided on a cladding of the semiconductor optical waveguide and whichis electrically connected to each other and an electrode which isprovided to face the grid-shaped electrodes via the semiconductoroptical waveguide in order to inject carriers into the semiconductoroptical waveguide.

The identifier is further constructed such that the encoding of theplurality of sets of tilted gratings is determined depending on whetheror not current is conducted to a plurality of the grid-shaped electrodesprovided along an optical axis.

The identifier is further constructed such that the optical beamfocusing means is a slab waveguide provided to guide the diffractedoptical beams and having a parabolic reflecting end face for reflectingthe guided optical beams, and the first photo receiver is provided at afocal point of the slab waveguide.

The identifier is further constructed such that the optical beamfocusing means comprises a second optical fiber for diffracting thediffracted optical beams to make the optical beams travel in claddingmode by using tilted gratings formed in a core portion of the secondoptical fiber, and the first photo receiver is provided at an end faceof the second optical fiber to receive the optical beams traveling incladding mode within the second optical fiber.

The identifier is further constructed such that the optical beamfocusing means further comprises a third optical fiber, located betweenthe optical waveguide and the second optical fiber, for focusing thediffracted optical beams from the optical waveguide onto a center of thesecond optical fiber.

The identifier is further constructed such that the optical waveguide isprovided on a front side of the third optical fiber and positioned aparta distance two times a focal length of the third optical fiber forfocusing, from the third optical fiber, and the second optical fiber isprovided on a rear side of the third optical fiber and positioned aparta distance two times a focal length of the third optical fiber from thethird optical fiber.

The identifier further comprises a fourth optical fiber in addition tothe third optical fiber, in which a center of the optical waveguide ispositioned at a front focal point of the third optical fiber, a centerof the second optical fiber is positioned at a rear focal point of thefourth optical fiber, and the third and fourth optical fibers focusdiffracted optical beams from the optical waveguide onto the secondoptical fiber.

The identifier is further constructed such that the optical beamfocusing means is a “fθ” lens provided so that a central axis of thecore of the optical waveguide and a light receiving face of the firstphoto receiver have relationship represented by “fθ”, and a tilt angleof each of the plurality of sets of tilted gratings is determined sothat a diffraction direction associated with diffracted optical beamsfrom each of the plurality of sets of tilted gratings satisfiesrelationship represented by “fθ”.

The identifier is further constructed such that light-diffractingefficiency of each of the plurality of sets of tilted gratings isdetermined so that intensity of diffracted optical beams from all of theplurality of sets of tilted gratings becomes uniform by controllingcurrent supplied to each of the plurality of grid-shaped electrodes.

An optical router according to the second aspect of the presentinvention is for switching between paths for a specific optical packetout of an optical signal consisting of an optical packet train having aplurality of optical packets coupled together, and the optical routercomprises: an optical branch for branching the optical packet traininput from an optical transmission input line; the optical packet headeridentifier defined in the first aspect of the present invention forreceiving one of optical outputs from the optical branch; an opticaldelay device for making the other of optical outputs from the opticalbranch delay by a predetermined time delay; an optical switch foroutputting at least one optical packet having a header identified by theoptical packet header identifier, the optical packet being separatedfrom the optical packet train output from the optical delay device, to afirst optical transmission output line, and outputting optical packetsexcluding the at least one optical packet identified by the opticalpacket header identifier to a second optical transmission output line,based on an output from the optical packet header identifier.

An optical router according to the third aspect of the present inventioncomprises a demultiplexer for demultiplexing thewavelength-division-multiplexed optical signal input from an opticaltransmission input line; a plurality of the optical routers defined inthe second aspect of the present invention for receiving a plurality ofoptical outputs having wavelengths different from one another from thedemultiplexer, respectively; and a multiplexer for multiplexing discretewavelength optical outputs from the second optical transmission line ofthe plurality of the optical routers defined in the second aspect of thepresent invention.

An optical router according to the fourth aspect of the presentinvention is an optical router having function of Optical Add/DropMultiplexer (Optical ADM) for switching between paths for a specificoptical packet out of a plurality of optical packets coupled togetherand constituting an optical packet train as an optical signal and, forinserting an optical packet different from the specific optical packetinto a location of the specific optical packet, the location becomingempty by switching between paths, and the optical router comprises: anoptical branch for branching the optical packet train input from a firstoptical transmission input line; an optical packet header identifier forreceiving one of optical outputs from the optical branch; an opticaldelay device for making the other of optical outputs from the opticalbranch delay by a predetermined time delay; and an optical switch foroutputting at least one optical packet having a header identified by theoptical packet header identifier, the optical packet being separatedfrom the optical packet train output from the optical delay device, to afirst optical transmission output line, outputting optical packetsexcluding the at least one optical packet identified by the opticalpacket header identifier to a second optical transmission output line,and inserting an optical packet from a second optical transmission inputline into a location of the at least one optical packet identified bythe optical packet header identifier, based on an output from theoptical packet header identifier.

An optical router according to the fifth aspect of the present inventionis an optical router having function of Optical Add/Drop Multiplexer(Optical ADM) for switching between paths for a specific optical packetout of a plurality of optical packets coupled together and constitutingan optical packet train, and inserting an optical packet different fromthe specific optical packet into a location of the specific opticalpacket, the location becoming empty by switching between paths, and theoptical router comprises: a demultiplexer for demultiplexing thewavelength-division-multiplexed optical signal input from the firstoptical transmission input line; a plurality of the optical routersdefined in the fourth aspect of the present invention for receiving aplurality of optical outputs having wavelengths different from oneanother from the demultiplexer, respectively; and a multiplexer formultiplexing discrete wavelength optical outputs from the plurality ofthe optical routers defined in the fourth aspect of the presentinvention into a wavelength division multiplexed optical signal andoutputting the multiplexed optical signal to the second opticaltransmission output line.

An optical routing method, according to the sixth aspect of the presentinvention, using the optical router defined in the fifth aspect of thepresent invention, comprises steps of: detecting a head of the opticalpacket train based on an output from the second photo receiver of theoptical packet header identifier; calculating, in the time domain,locations of headers of optical packets constituting an optical packettrain based on the time when the head of the optical packet train isdetected; detecting an output from the first photo receiver of theoptical packet header identifier at individual times corresponding tothe locations; and making a corresponding optical packet delay by a timeperiod in order to separate the corresponding optical packet from theoptical packet train input to the optical router, the time period beingequal to a time delay by which the optical packet train is made to delayby the optical delay circuit to transmit to the optical switch a controlsignal so that the optical switch changes its switch state for aduration of the corresponding optical packet, in the event the detectedoutput from the first photo receiver is higher than a predeterminedlevel.

The optical packet header identifier of the present invention comprises:an optical waveguide configured to have a plurality of sets of tiltedgratings, the locations of which are previously encoded in a directionalong an optical axis, arranged along the waveguide; light-focusingmeans; and a photo receiver. Accordingly, the optical packet headeridentifier having no necessity for calculation of correlation betweencodes performed by an electronic circuit and a low power consumption,operating at high rate, a superior reliability, stability and economicalefficiency can be achieved.

Furthermore, the optical packet header identifier of the presentinvention can be configured to be programmable as follows. That is, theoptical waveguide comprises a semiconductor optical waveguide, theplurality of sets of tilted gratings are realized such that a pluralityof sets of grid-shaped electrodes are formed on the semiconductoroptical waveguide and at least one of the plurality of sets ofgrid-shaped electrodes is previously selected in response to a code tobe identified, and then is supplied with current to produce at least oneset of tilted gratings within the semiconductor optical waveguide byutilizing a band-filling effect.

Moreover, the optical packet header identifier makes it possible toconstruct an optical ADM and an optical router each incorporating theidentifier therein and having a simplified configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantage of the presentinvention will become apparent from the following detailed descriptionwhen taken with the accompanying drawings in which:

FIG. 1 is a diagram illustrating the configuration of a conventionaloptical packet header identifier;

FIG. 2 is a diagram illustrating the configuration of a first embodimentof an optical packet header identifier according to the presentinvention;

FIG. 3 is a diagram illustrating the configuration of a first example ofthe first embodiment of the optical packet header identifier accordingto the present invention and FIG. 3A is a plan view of the first exampleof the first embodiment, and FIG. 3B is a side view thereof;

FIG. 4 is a diagram illustrating the configuration of a second exampleof the first embodiment of the optical packet header identifieraccording to the present invention and FIG. 4A is a plan view of thefirst example of the first embodiment, and FIG. 4B is a side viewthereof;

FIG. 5 is a diagram illustrating the configuration of a third example ofthe first embodiment of the optical packet header identifier accordingto the present invention;

FIG. 6 is a diagram illustrating the configuration of a fourth exampleof the first embodiment of the optical packet header identifieraccording to the present invention;

FIG. 7 is a diagram illustrating the configuration of a fifth example ofthe first embodiment of the optical packet header identifier accordingto the present invention and FIG. 7A is a plan view of the first exampleof the first embodiment, and FIG. 7B is a side view thereof;

FIG. 8 is a diagram illustrating the configuration of a sixth example ofthe first embodiment of the optical packet header identifier accordingto the present invention and FIG. 8A is a plan view of the first exampleof the first embodiment, and FIG. 8B is a side view thereof;

FIG. 9 is a diagram illustrating the configuration of a secondembodiment of an optical packet header identifier according to thepresent invention;

FIG. 10 is a diagram illustrating the configuration of a firstembodiment of an optical network node having the optical packet headeridentifier of the present invention applied thereto;

FIG. 11 is a diagram illustrating the configuration of a secondembodiment of the optical network node having the optical packet headeridentifier of the present invention applied thereto; and

FIG. 12 is a diagram illustrating the configuration of a thirdembodiment of the optical network node having the optical packet headeridentifier of the present invention applied thereto.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a diagram illustrating the general configuration of an opticalpacket header identifier according to the present invention. The opticalpacket header identifier comprises an optical waveguide 10, opticalfocusing means 15, and photo receivers 17 and 20, and tilted gratings 11for diffracting an incident optical beam 12 to emit the beam as anemitted optical beam 14 to the outside of the waveguide is formed withinthe optical waveguide 10. The tilted gratings are not formed uniformlyalong the optical waveguide 10, but are constructed such that aplurality of sets of tilted gratings are arranged at intervals. Thelength of a portion containing a set of tilted gratings and the lengthof a portion containing no gratings, can be defined in increments oflength “L.” “L” equals to the length that 1-bit in a digital opticalsignal 12 occupies within the waveguide 10. Arrangement of the pluralityof sets of tilted gratings along an optical axis coincides with apreviously given address code. As shown in FIG. 2, an 8-bit codeincluding the binary sequence “10100101” is provided within thewaveguide. The optical signal 12 input to the waveguide travels withinthe waveguide 10 and at the moment when arrangement of the code of theoptical signal and arrangement of the plurality of sets of tiltedgratings spatially coincide with one another, the optical beam guidedwithin the waveguide is diffracted to the outside of the waveguide,while having its maximum intensity. The optical focusing means 15performs the spatial integration of the emitted optical beams 14, whichare emitted to the outside of the waveguide by the plurality of sets oftilted gratings, and then the optical beams 14 are output as a focusedoptical beam 16 to the photo receiver 17. When the photo receiver 17 ismade to have a bandwidth so that the receiver serves as a low-passfilter for making the frequency of a clock signal input theretoapproximately be the cut-off frequency thereof, the photo receiver 17performs the integration of the focused optical beams 16 for a timeperiod, and converts the beam to an electrical signal, and then outputsan electrical output signal 18.

The optical packet header identifier outputs a correlation calculationresult 19 as an electrical signal by correlating an optical signal 13input to the waveguide and encoded through intensity modulation with thepattern of the plurality of sets of tilted gratings that are previouslyarranged. As shown in FIG. 2, when arrangement of the optical signal 13input to the waveguide and the pattern 11 of the plurality of sets oftilted gratings coincide with one another, a correlation signal 19having a high peak value is output, and when those two components, i.e.,arrangement and pattern, do not coincide with one another, a peak doesnot appear in the signal 19 because of low correlation between them. Theabove-described operation makes it possible to detect the address codeadded to the optical signal. The photo receiver 20 receives an opticalpacket train transmitting through the optical waveguide 10. Aphotoelectrically converted signal 21 output from the waveguide is usedfor producing a timing signal indicative of locations in headers ofindividual packets that constitute the optical packet train to introducethe correlation signal 19, and is used for producing a timing signal toseparate the optical packets, all of which have the address addedthereto.

Assuming that the waveguide is a quartz waveguide containing Ge and hasa refractive index of 1.45, and the wavelength of optical signal is 1.55μm, and further a bit rate is 40 Gbps, the length “L” along thewaveguide, which length corresponds to 1-bit in an optical signal, is2.6 mm, and the length that corresponds to 8-bit in a signal is about 21mm. The tilted gratings are formed to have a periodic interval of 0.76μm, provided that the gratings diffract the incident optical beam 12 ina direction vertical to an optical axis, and then produce the emittedlight 14. In case of a fiber Bragg reflector for optical communication,gratings are formed therein for practical use to have a periodicinterval of about 0.5 μm. Therefore, the previously described gratingscan be formed using a well-known technique for forming a fiber gratingthrough interference generated, using a phase grating, between opticalbeams having a short wavelength, such as KrF excimer excitation light.

When the waveguide is a semiconductor with a refractive index of 3.5,the length that 8-bit in a signal occupies within the waveguide canfurther be shortened to 8.7 mm, and the tilted gratings can be formed tohave a periodic interval of 0.3 μm, meaning that a desired waveguide caneasily be fabricated by a lithography technique.

A detailed embodiment of the above-described optical packet headeridentifier and examples describing the embodiment in more detail will bedisclosed below. In a first embodiment, the optical waveguide 10depicted in the general diagram of the present invention shown in FIG. 2comprises an optical fiber, and the optical focusing means 15 isrealized by the following examples 1 through 6. Though not shown, theconfigurations of the examples all include the photo receiver 20.

FIG. 3 illustrates a first example showing an optical packet headeridentifier comprising a single-mode optical fiber 110 for communication,a planar slab waveguide 115 having a reflecting plane, whose outline isparabolic, in a plane thereof and a photo receiver located at a focalpoint P on a parabolic curve, and FIG. 3A is a plan view of theidentifier, and further, FIG. 3B is a side view thereof. In a core ofthe optical fiber 110, tilted gratings 111 are formed such thatlocations of the plurality of sets of tilted gratings along an opticalaxis are encoded.

When an incident optical beam 12 having an address added thereto throughintensity modulation travels within the optical fiber, the beam isdiffracted by the tilted gratings in a direction approximately verticalto the optical axis of the core. The diffracted optical beams areincident on the slab waveguide 115, and travel while being totallyreflected by both the primary principal surfaces of the waveguide, andthen, are focused onto a focal point “P” as is seen when using aparabolic antenna because a plane of the waveguide facing the incidentplane thereof has a parabolic reflecting surface, and further thespatial integration of the focused optical beams is performed. Disposedat the focal point “P” is a photo receiver 17 for outputting acorrelation signal as an electrical output signal 18 indicative of acorrelation between the address code added to an optical signal and thespatially-encoded arrangement of the plurality of sets of tiltedgratings by using ability of photo receiver to perform time integrationand photoelectric conversion function thereof.

Thickness of the slab waveguide may be determined such that a diffractedoptical beam leaving the core of the optical fiber and expanding in adirection vertical to the plane of this figure can entirely be capturedby the slab waveguide when the beam reaches the end face of the slabwaveguide. Since an optical beam is guided nearly exhibiting a Gaussiandistribution within an optical fiber, the thickness T of the slabwaveguide may be represented by relationship:T>2W=2λz/(πw ₀),where: “z” is a distance in air from the core to the linear side face ofthe slab waveguide; “2w” is a beam size (entire width) at the distance“z”; “2w₀” is a beam size (entire width) when the beam propagates in thesingle-mode optical fiber 110; and “λ” is a wavelength. When wavelengthis 1.55 μm and the side face of the optical fiber and the linear sideface of the slab waveguide contact one another, and further when takinginto account the assumption that “2w₀” is about 6 μm in case of thesingle-mode fiber, the beam size “2w” at the end face of the slabwaveguide on which optical beams are incident is about 30 μm, andaccordingly the waveguide may need to have a thickness in the range from30 to 50 μm.

Furthermore, a reflective coating material may be applied to theparabolic plane if necessary, or selecting a parabolic function of theparabolic plane also makes it possible for all of optical beams tototally be reflected on the parabolic plane.

FIG. 4 illustrates a detailed second example of the optical packetheader identifier according to the present invention. The identifier ofthe second example comprises a single-mode optical fiber 110 in which aplurality of sets of tilted gratings 111 are formed such that locationsof the plurality of sets of tilted gratings are encoded along an opticalaxis, a multi-mode optical fiber 310 in which tilted gratings 311 areformed uniformly within the core of the fiber along an optical axis, afocusing optical fiber 210 serving as a cylindrical lens for focusingdiffracted and emitted optical beams from the plurality of sets oftilted gratings 111 of the single-mode optical fiber 110 onto the coreof the multi-mode optical fiber 310, and a photo receiver 17 located atthe output end of the multi-mode optical fiber 310. FIG. 4A is a planview of the identifier, and FIG. 4B is a side view thereof. It should beappreciated that the example employs the focusing optical fiber 210 andthe multi-mode optical fiber 310 instead of the optical focusing means15 shown in FIG. 2.

The tilted gratings 311 of the multi-mode optical fiber 310 areconfigured to make optical beams focused through and input from thefocusing optical fiber 210 diffracted to travel in cladding mode withinthe multi-mode optical fiber 310. In more detail, the optical fibers 110and 310 are made of the same material, and different only in the corediameter. The plurality of sets of tilted gratings 111, the locations ofwhich are encoded within the fiber to correspond to an address code, andthe tilted gratings 311 formed uniformly within the core of the fibereach have gratings formed at the same periodic intervals and tilted atangles slightly different from one another. The plurality offsets oftilted gratings 111 are configured to have gratings tilted at a specificangle relative to the optical axis of the core in order to diffract anoptical beam guided along the fiber in a direction approximatelyvertical to the optical axis. The tilted gratings 311 uniformly formedalong the core of the fiber 310 are configured to have gratings tiltedat a specific angle relative to the optical axis of core in order todiffract optical beams incident vertically thereon not in parallel withthe optical axis of core, but at an angle slightly distorted such thatthe optical beams travel in cladding mode within the optical fiber 310after being diffracted. This is because when optical beams arediffracted in parallel with the optical axis of core, the beams arere-diffracted within the tilted gratings 311. In the optical fiber 310,the cross section of clad is far larger than that of the core, andtherefore optical beams traveling in cladding mode within the fiber areless re-diffracted by gratings formed in the core. This allows opticalbeams traveling in cladding mode to propagate within the cladding of theoptical fiber 310 without experiencing any transmission losses due tothe uniformly formed gratings 311, and transfer their entire power tothe photo receiver provided at the output end.

The focusing optical fiber 210 serving as a cylindrical lens is disposedsuch that the cores of the optical fibers 110 and 310 are positionedrespectively at locations apart a distance in air two times focal lengthfrom the principal plane of the fiber 210, in order to couple opticalbeams in the same optical magnification. Focal length “f” of thecylindrical lens having a refractive index “n” and an entire circle ofradius “r” is represented by the following equation.1/f=2·(n−1)/r−2·(n−1)² /n·rWhen the focusing optical fiber 210 is realized by employing a silicaoptical fiber that has a refractive index of 1.45 and a diameter “2r” of125 μm, “f” becomes equal to 1.61r and thereby, is 100.6 μm.Accordingly, the optical fibers 110, 310 and the focusing optical fiber210 may be disposed such that spacing between centers of the cores ofthe optical fiber 110 and the focusing optical fiber 210, and spacingbetween centers of the cores of the optical fiber 310 and the focusingoptical fiber 210 each become 201.2 μm in air.

In the above-described second example, although the focusing opticalfiber 210 is optically disposed so that the beams emitted from theoptical fiber 110 are focused onto the optical fiber 310 in the sameoptical magnification, optical fibers 410-1 and 410-2 may be employed astwo cylindrical lens, as shown in a third example depicted in FIG. 5, inwhich emitted optical beams from the fiber 110 are once made to becollimated optical beams in a plane vertical to the paper by the firstoptical fiber, and then the collimated optical beams are focused throughthe second optical fiber, thereby constituting an optical infinitearrangement.

Moreover, as shown in a fourth example depicted in FIG. 6, theidentifier of the invention may be constructed such that the focusingoptical fiber 210 is omitted and cylindrical faces of the optical fiber110 for diffraction of light and the optical fiber 310 for reception oflight are disposed near one another, and as a result, the slab waveguideof the first example having a parabolic reflecting end face is replacedby the optical fiber 310 for reception of light.

Location at which the optical fiber 310 for reception of light is to bedisposed and core diameter that the optical fiber 310 for reception oflight is to have for receiving all of diffracted optical beams from theoptical fiber 110 will be calculated. The calculation can be performedby using a focusing formula applied to the case where optical beams arefocused through one of side faces of the optical fiber 310 for receptionof light. A distance “d” between center of the core of the optical fiber110 for diffraction of light and cylindrical face of the optical fiber310 for reception of light, which distance is determined such thatdiffracted optical beams from the optical fiber 110 for diffraction oflight travel the distance “d” and after traveling through the curvedface of the optical fiber 310 for reception of light, are collimatedwithin the optical fiber 310 for reception of light, becomes about 140μm in air. When making a distance between the two fibers larger thanthis value, the diffracted optical beams are focused within the opticalfiber 310. When the distance “d” is determined so that the diffractedoptical beams from the optical fiber 110 collimated within the opticalfiber 310, a height 2w of the diffracted optical beams from the opticalfiber 110 for diffraction of light becomes about 45 μm at the side faceof the optical fiber 310, provided that the optical beams within thecore of the optical fiber 110 for diffraction of light nearly areGaussian beams having a diameter of 2w₀=6 μm. Accordingly, the core ofthe optical fiber 310 for reception of light may have a diameter ofabout 50 μm.

An advantage of the example is that, even when bit rate of opticalsignal is low and/or length of address code becomes long, the elementnever becomes longitudinally large in size. For example, when the bitrate of signal is 1 Gbps and the address code has 32 bits in length, theoptical fibers 110, 310 disposed near one another should have a longlength of 3.2 m. However, the optical fibers 110, 310 having such longlength can be mounted in a small space by winding the two fibers in theform of a coil or reel.

Furthermore, the above-described bit rate of signal corresponds to thebit rate used in LAN such as Ethernet (registered trademark) or anaccess system. This means that the present invention can effectively beused even at a bit rate of signal at which a terminal located at thetermination of network operates.

A fifth example is illustrated in FIG. 7. FIG. 7A is a plan view andFIG. 7B is a cross sectional view taken along a central line of the planview.

The optical packet header identifier of the example comprises an opticalwaveguide 510 having a circular optical axis, a cylindrical lens 520depicted as a circular arc in the same plane as the optical waveguide510, a normal focusing lens 530 having axisymmetric focusing power, anda photo receiver 17. It should be appreciated that the example employsthe cylindrical lens 520 and the focusing lens 530 instead of thefocusing means 15 shown in FIG. 2.

The optical waveguide 510 has a plurality of sets of tilted gratings 511provided in a core thereof such that the locations of the plurality ofsets of tilted gratings 511 are encoded along the optical axis withinthe waveguide. Each set of tilted gratings 511 are tilted at an anglesuch that a central axis of diffracted optical beams emitted from theeach set of tilted gratings passes through a point Q shown in the planview of FIG. 7A. As shown in FIG. 7B, which is the cross section cut bya plane vertical to the paper, the cylindrical lens 520 is disposed tohave its focal point at the center of the core of the optical waveguide510 in order to collimate optical beams emitted from the tilted gratingsand expanding therefrom, within the cylindrical lens 520. Furthermore,the cylindrical lens 520 is depicted as a circular arc centering thepoint “Q” so as not to cause optical aberration in the collimatedoptical beams emitted from each of the plurality of sets of tiltedgratings 511. The focusing lens 530 having axisymmetric focusing powerfocuses an image formed at the point “Q” onto the photo receiver 17.

A sixth example is illustrated in FIG. 8. FIG. 8A is a plan view andFIG. 8B is a cross sectional view taken along a central line of the planview.

The optical packet header identifier of the example comprises alinear-shaped optical waveguide 110, a focusing lens 630 havingdifferent lens characteristics in a horizontal plane and a planevertical to the horizontal plane, and a photo receiver 17. The exampleemploys only the focusing lens 630 instead of the focusing means 15shown in FIG. 1.

The optical waveguide 110 has a plurality of sets of tilted gratings 611provided in a core thereof so that the locations of the plurality ofsets of tilted gratings are encoded along the optical axis within thewaveguide. Each of the plurality of sets of tilted gratings 511 aretilted at an angle such that a distance “t” from a central position “R”of the optical axis of optical fiber to a central position of each ofthe plurality of sets of tilted gratings in a longitudinal direction andan angle “θ” at which an optical beam incident on the waveguide isdiffracted by the corresponding set of tilted gratings satisfy therelationship represented by t=fθ. In this case, “f” represents a focallength within a horizontal plane of the focusing lens 630.

The focusing lens 630 acts as a “fθ” lens within the horizontal planeand as a cylindrical lens to focus optical beams emitted and expandingfrom the plurality of sets of tilted gratings 611 within the core of theoptical waveguide 110, onto the photo receiver 17 within the verticalplane. A lens performing different lens operations in planes orthogonalto one another can be realized by, for example, employing a compositelens composed of an “fθ” lens and a cylindrical lens, a hologram lens,an aspherical lens, etc. Furthermore, instead of lens, an asphericalmirror may be employed.

Although in the embodiments described so far, as the optical fibers 110,510 each have a plurality of sets of tilted gratings provided therein sothat the locations of the plurality set of tilted gratings are encoded,a single-mode optical fibers for communication are employed, but theembodiments need not to be limited to employment of single-mode opticalfiber, but may employ a multi-mode optical fiber.

Moreover, material of fiber employed in the invention is not limited toa silica, but may be a glass or plastic. Additionally, the core employedin the invention is not limited to a circular core, but may be arectangular core or a channel optical waveguide formed on a planarsubstrate.

In addition, the focusing means employed in the invention is not limitedto an optical fiber and a planar waveguide, and further, material of thefocusing means is not limited to a glass and plastic.

To make a correlation signal output from the photo'receiver 17 shown inFIG. 2 output a clear correlation peak that has a symmetrical waveformand exhibits no deformation of waveform in the time domain, intensitiesof diffracted optical beams from all the tilted gratings 11 need to beuniform. When the intensities of diffracted optical beams along theoptical axis of the optical waveguide 10 are significantly differed, thewaveform of correlation signal deforms and then a peak indicative ofclear correlation may not appear. When diffraction efficiency of theplurality of sets of tilted gratings is low, the amount of optical powerattenuated in proportion to a distance that the incident optical beam 12propagates along the optical axis of the optical waveguide 10 is small,and diffracted optical beams from any one of the plurality of sets oftilted gratings are output, having approximately equal intensity.

In the event the above-described diffraction efficiency is maderelatively high to increase the quantity of light to be received by thephoto receiver 17, the previously mentioned fault will occur. If thediffraction efficiency is made different depending on the locations ofthe plurality of sets of tilted gratings, the fault can be avoided. Inmore detail, a desired optical waveguide is constructed such thatdiffraction efficiency of sets of tilted gratings located near the inputend of the waveguide for an incoming optical beam is made low anddiffraction efficiency of sets of tilted gratings located near theoutput end thereof for an outgoing optical beam is made high, therebyallowing intensity of diffracted optical beams from any one of theplurality of sets of tilted gratings to substantially be the same. Theabove-described desired optical waveguide can be realized by controllingintensity of ultraviolet ray (UV ray) to be irradiated, the number ofpulses in UV ray, and/or time period over which UV ray is beingirradiated, at the time of formation of fiber grating.

Furthermore, in the second to fourth examples, each employing as anoptical focusing member an optical fiber for reception of light in whichgratings are formed, the optical fiber 310 for reception of light may bemade to have weighed diffraction efficiency along the optical axis, orthe optical waveguide may be made together with the optical fiber 310.

A second embodiment of the optical packet header identifier according tothe present invention will be explained below. Based on the conceptshown in FIG. 2, the second embodiment employs a semiconductor opticalwaveguide as the optical waveguide 10, and realizes gratings by formingin the semiconductor optical waveguide spatial periodicity of refractiveindex variation, which periodicity is generated so that refractive indexwithin the waveguide is lowered by performing spatial periodic currentinjection (carrier injection) into the waveguide so that the injectionacts through a band-filling effect on a guided optical beam. Theband-filling effect means a phenomenon in which empty energy levelswithin an energy band are filled by injecting of free carriers to shiftwavelength at absorbance edge substantially to the side ofshort-wavelength, thereby lowering refractive index of semiconductor, asis known to those skilled in the art. The lowering of refractive indexis independent on polarized wave. Focusing means and two photo receiversof the embodiment are configured to have the configuration similar tothat of the first embodiment.

FIG. 9 illustrates an optical waveguide which is constructed such thatburied-channel optical waveguide of “p-i-n” structure is formed throughetching and growth of crystalline on a transparent semiconductorsubstrate and a plurality of sets of electrodes are disposed thereon inthe form of duckboard, and forward current is injected between selectedset of electrodes and a ground electrode located on a back face of thesubstrate. Formed within channel waveguide below selectedduckboard-shaped set of electrodes are periodic refractive indexgratings through the above-described band-filling effect. The periodicrefractive index gratings are not formed below unselected set ofelectrodes. The concept of the present invention shown in FIG. 2 isrealized by selecting sets of electrodes to be conducted current so thatthe selected sets of electrodes corresponds to an address code. Thisindicates that the second embodiment has a significant advantage overthe first embodiment in that although a code produced by correspondingsets of tilted gratings is fixed in the first embodiment, a codeproduced by the same is programmable in the second embodiment.

A semiconductor optical waveguide 710 having programmable chirp gratingstherein is formed such that an n-type cladding layer 713, an i-layer 714as a core, a p-type cladding layer 715 and a p-type contact layer 716 ona n-type semiconductor substrate are laminated through crystal growthtechnique in a planar form and both sides of the laminated layers areetched to expose the substrate so that the remaining laminated layersbecome a channel waveguide, and the channel waveguide is buried in theoptical waveguide by filling both sides of the channel waveguide with aninsulative component 717 having a low carrier concentration. Thereafter,a plurality of sets of duckboard-shaped electrodes 711-1 to 711-5 areformed on the waveguide, and a ground electrode 719 is formed over theback face of the substrate. Two sets of duckboard-shaped electrodesadjacent to one another are electrically isolated from one another.Spacing between grids formed within each of the plurality of sets ofduckboard-shaped electrodes and an angle at which a guided optical beampropagates relative to an optical axis are determined such thatrefractive index gratings formed within the optical waveguide byconducting current between a set of electrodes and the back faceelectrode 719 diffract an incident optical beam 718 in a directionapproximately orthogonal to the waveguide to produce diffracted opticalbeams 14.

Length “L” of one set of duckboard-shaped electrodes along the opticalaxis is determined to be equal to the length that 1-bit of a signal inthe incident optical beam 718 within the semiconductor crystalline. Setsof electrodes to be conducted current are selected based on an addresscode in the header portion of an optical signal to be detected. In FIG.9, the address code is represented by a 5-bit code including the binarysequence “10010”, and the sets of electrodes 711-1 and 711-4 areselected correspondingly. That is, refractive index gratings are formedonly below the two sets of electrodes 711-1 and 711-4 through aband-filling effect.

By combining the semiconductor optical waveguide 710, the opticalfocusing member 15 having detailed configuration shown in theabove-described examples 1 through 7, and the photo-receiver 17together, only when an incoming optical packet 718 has a header code,including the binary sequence “10010,” added thereto, the correlationsignal shown in FIG. 2 is output from the photo receiver 17. As isdescribed above, since the second embodiment has ability to optionallyselect sets (or a set) of electrodes to be conducted current, a headercode to be identified can optionally be determined, whereby aprogrammable optical packet header identifier can be constructed.

Additionally, adjusting the amount of current to be supplied tocorresponding sets of electrodes easily allows intensity of diffractedoptical beams to be uniform along an optical axis.

It should be appreciated that an optical semiconductor material to beemployed can be realized by selecting a most effective material, such asSi series, InP series, GaAs series and AlN series, depending on thewavelength of an optical signal to be used.

An embodiment of an optical network incorporating therein the opticalpacket header identifier of the present invention will be disclosedbelow.

FIG. 10 is a diagram illustrating the configuration of an OpticalAdd/Drop Multiplexer (OADM) that incorporates therein the optical packetheader identifier of the present invention. An OADM 800 comprises anoptical packet header identifier 801 of the present invention, a 2×2optical switch circuit 802, an optical switch control circuit 803, anoptical branch 804, and an optical delay circuit 805.

An incident optical signal 806 coupling a number of packets together andinput to the OADM 800 is branched into two signals by the optical branch804, and then one of the two signals is input to the optical packetheader identifier 801, whether the other is delayed by the optical delaycircuit 805, thereby being input to the 2×2 optical switch circuit 802.The optical packet header identifier 801 calculates correlation betweenan address code previously given to the identifier and an optical packetinput to the identifier, and then outputs a correlation signal 818having the waveform 19 shown in FIG. 2 and a signal 821 produced byconverting an optical packet signal which transmits through the opticalwaveguide 11 shown in FIG. 2 to an electrical signal.

The optical switch control circuit 803 detects a head of an opticalpacket train from the photoelectrically converted packet signal 821.Locations in the time domain at which the headers of individual packetsare located are identified based on the time when the head of the packettrain has been detected, time windows are periodically created, and thenthe correlation signal 818 is captured at individual times correspondingto the time windows. Upon detection of the correlation signals, in orderto separate all the packets, whose correlation signals were detected,from the packet train input to the identifier, the optical switchcontrol circuit 803 outputs a control signal 815 to the 2×2 opticalswitch circuit 802, so that the 2×2 optical switch circuit 802, which isnormally in a pass-through switch state, is delayed by the time delay bywhich the corresponding packets are delayed by the optical delay circuit805, and changes its state, and then stays in a cross switch state forthe duration of the corresponding packet. This allows only the packets,which are to be separated through the OADM 800 at this node and from anoptical packet train 809 passing through the optical delay circuit 805,to be output to a separation output 810, and the remainder of the packettrain is output to a transmission output line 812 after passing throughthe OADM because the 2×2 optical switch circuit 802 is in a pass-throughswitch state. When another packet is inserted through this node into anempty time slot, in which the corresponding separated packet waslocated, a packet to be inserted is input from an input port 811,creates a new optical packet train along with other packets, which wereinput through the input 806 and not separated at this node, and finally,is output to the transmission output line 812.

In the case where the separation output 810 does not terminate at thisnode, but is transmitted to a transmission output line different fromthe transmission output line 812, the node 800 comes to operate as arouter.

The 2×2 optical switch circuit 802 that operates at high rate may be awaveguide optical switch that uses lithium niobate crystalline having aferroelectric crystalline. In addition, the optical delay circuit 805may be an optical fiber delay line.

FIG. 11 illustrates an embodiment in which an Optical Add/DropMultiplexer (OADM) node in a Wavelength-Division Multiplexing (WDM)transmission system is constructed by employing the optical packetheader identifier of the present invention.

A WDM-OADM comprises a demultiplexer 901 for demultiplexing wavelengthdivision multiplexed (WDM) optical signals input from a primarytransmission line 906, a plurality of OADMs 800-1 to 800-n which areprovided such that n pieces of the OADMs shown in FIG. 9 are arranged inparallel with one another so as to correspond to individual wavelengths,and a multiplexer 902 for multiplexing optical signals passing throughthe OADMs and a packet newly inserted at this node to create a newpacket train, and for transmitting the new packet train to a primarytransmission line on the side of output. How an OADM operates is thesame as that explained in the description of FIG. 10.

In the embodiment, in order for the WDM-OADM to be able to securelymultiplex and demultiplex optical signals even when optical signalshaving different wavelengths are not in synchronization with one anotherin the time domain, the OADMs corresponding to individual wavelengthseach include the optical packet header identifier of the presentinvention.

The optical packet header identifier 801 is configured to detect oneaddress code in the above-stated explanation. However, in order for theoptical packet header identifier to be able to detect different multipleaddress codes, the optical waveguide 10 constituting the optical packetheader identifier may be configured to have a plurality of waveguidesconnected in series, so that each of the plurality of waveguidescontains a plurality of sets of tilted gratings, the locations of whichare encoded in accordance with a code different from other codes.

Furthermore, in the case where a separation output from each of theOADMs 800-1 to 800-n does not terminate at this node, but is transmittedto a transmission output line different from the transmission outputline 912 after being multiplexed, the node 800 comes to operate as arouter.

FIG. 12 illustrates the configuration of WDM-OADM in whichwavelength-multiplexed optical packet trains are in synchronization withone another in the time domain, and packet length of each of packetsthat constitute a packet train is the same. In this case, individualOADMs corresponding to individual wavelengths need not to have theoptical packet header identifier provided therein, but can share oneoptical packet header identifier.

The WDM-OADM of the embodiment comprises an optical branch 804 forbranching all together optical signals inserted into and transmittedthrough the primary optical transmission line 906, and corresponding toa number of wavelengths, an optical packet header identifier 801 towhich one of the branched optical beams is input, and further which isconstructed by coupling in series optical packet header identifierscorresponding to different wavelengths, an optical delay circuit 805 bywhich the other of the branched optical beams is delayed, an opticaldemultiplexer 901 for demutiplexing wavelength division multiplexedoptical signals passing through the optical delay circuit 805, a 2×2optical switch circuit 802 to which each of demultiplexed opticalsignals corresponding to individual wavelengths is input, an opticalmultiplexer 902 for multiplexing discrete wavelength optical beamsoutput from the 2×2 optical switch circuit 802, and transmitting themultiplexed beams to a primary transmission output line 912, and anoptical switch control circuit 813.

Optical packet header identifiers 801-1 to 801-n are constructed suchthat the optical waveguide 10 shown in FIG. 2 constitutes a plurality ofoptical waveguides, which are coupled in series along an optical axisand each of the plurality of optical waveguides has a plurality of setsof tilted gratings arranged therein, encoded to correspond to anassociated code, and corresponding to a wavelength different from theremaining wavelengths, and in addition, are provided means for focusingand receiving individual diffracted optical beams corresponding todifferent wavelengths. The photo receiver 20 shown in FIG. 2 is singlyemployed in the embodiment for receiving a packet train that containsoptical beams corresponding to a plurality of wavelengths and packetsprovided in the same format and being in synchronization with oneanother. Although operation of the WDM-OADM of the embodiment is similarto that explained in the description of the embodiment shown in FIG. 10,the optical switch control circuit 813 transmits a control signal foropening/closing switch to a plurality of 2×2 optical switch circuits802-1 to 802-n.

The embodiment also detects a plurality of address codes using onewavelength, and then separates corresponding packets (or a correspondingpacket). This operation is similar to that explained in the descriptionof the embodiment shown in FIG. 11.

Furthermore, in the case where a separation output from each of the 2×2optical switch circuits 802-1 to 802-n does not terminate at this node,but is transmitted to a transmission output line different from thetransmission output line 912 after being multiplexed, the node 800 comesto operate as a router.

As described so far, although an optical ADM has been described as anexample of an optical network node that employs the optical packetheader identifier of the present invention, instead of optical ADM, anOptical Cross Connect (OXC) may be also employed. The special packetheader identifier of the present invention can be used in a system forrouting optical packets over a wide area while maintaining the packetsin the form of light.

While the present invention has been described in connection withcertain preferred embodiments, it is to be understood that the subjectmatter encompassed by the present invention is not limited to thosespecific embodiments. On the contrary, it is intended to include allalternatives, modifications, and equivalents as can be included withinthe spirit and scope of the following claims.

1. An optical packet header identifier for identifying a header of anoptical packet, comprising: an optical waveguide, a tilted grating fordiffracting an optical beam propagating within a core of said opticalwaveguide toward an outside of said optical waveguide, a set of tiltedgratings constituted by said tilted grating and having a thickness ofgratings approximately equal to a length occupied in a direction alongan optical axis within said optical waveguide by one bit of a signal ofan optical packet, and said plurality of sets of tilted gratings areencoded and arranged in a direction along an optical axis based on aspecific header code for said optical packet; optical beam focusingmeans for spatially focusing optical beams diffracted by all of the setsof tilted gratings; a first photo receiver for receiving said focusedoptical beams; and a second photo receiver for receiving an optical beampropagating through said optical waveguide at an output end of saidoptical waveguide.
 2. The optical packet header identifier according toclaim 1, wherein said optical waveguide is any one of an optical fiberand a channel waveguide formed on a planar substrate.
 3. The opticalpacket header identifier according to claim 2, wherein said optical beamfocusing means comprises a second optical fiber for diffracting saiddiffracted optical beams to cladding mode by using tilted gratingsformed in a core portion of said second optical fiber, and wherein saidfirst photo receiver is provided at an end face of said second opticalfiber to receive said optical beams traveling in cladding mode withinsaid second optical fiber.
 4. The optical packet header identifieraccording to claim 3, wherein said optical beam focusing means furthercomprises a third optical fiber located between said optical waveguideand said second optical fiber, for focusing said diffracted opticalbeams from said optical waveguide onto a center of said second opticalfiber.
 5. The optical packet header identifier according to claim 4,wherein said optical waveguide is provided on a front side of said thirdoptical fiber, and positioned apart a distance two times a focal lengthof said third optical fiber for focusing from said third optical fiber,and wherein said second optical fiber is provided on a rear side of saidthird optical fiber, and positioned apart a distance two times a focallength of said third optical fiber from said third optical fiber.
 6. Theoptical packet header identifier according to claim 4, furthercomprising a fourth optical fiber, wherein a center of said opticalwaveguide is positioned at a front focal point of said third opticalfiber, wherein a center of said second optical fiber is positioned at arear focal point of said fourth optical fiber, and wherein said thirdand fourth optical fibers focus diffracted optical beams from saidoptical waveguide onto said second optical fiber.
 7. The optical packetheader identifier according to claim 2, wherein said optical beamfocusing means comprises a “fø” lens provided such that a central axisof said core of said optical waveguide and a light receiving face ofsaid first photo receiver have relationship represented by “fø”, andwherein a tilt angle of each of said plurality of sets of tiltedgratings is determined such that optical beams diffracted from each ofsaid plurality of sets of tilted gratings are focused onto said photoreceiver by said “fø” lens.
 8. The optical packet header identifieraccording to claim 2, wherein said optical waveguide is provided to forma circle in a plane, said plane having said tilted grating tiltedtherein, and wherein said optical beam focusing means comprises: anoptical fiber acting as a cylindrical lens and provided such that saidoptical fiber forms another circle around the same center as that ofsaid circle in the same plane as that for said optical waveguide to formsaid circle and a center of said core of said optical waveguide ispositioned at a front focal point of said optical fiber; a lens forfocusing optical beams transmitted from said optical fiber onto saidfirst photo receiver.
 9. The optical packet header identifier accordingto claim 2, wherein light-diffracting efficiency of each of saidplurality of sets of tilted gratings is determined such that intensityof diffracted optical beams from all of said plurality of sets of tiltedgratings becomes uniform.
 10. The optical packet header identifieraccording to claim 1, wherein said optical beam focusing means comprisesa slab waveguide provided to guide said diffracted optical beams andhaving a parabolic reflecting end face for reflecting said guidedoptical beams, and said first photo receiver is provided at a focalpoint of said slab waveguide.
 11. The optical packet header identifieraccording to claim 1, wherein said optical waveguide comprises asemiconductor optical waveguide, and wherein said tilted gratingcomprises a spatial periodic refractive index variation generated insaid core by a band-filling effect, said effect being caused whencarriers are injected into said semiconductor optical waveguide, andwherein said set of tilted gratings is formed by conducting currentbetween grid-shaped electrodes which is provided on a cladding of saidsemiconductor optical waveguide and which is electrically connected toteach other and an electrode which is provided to face said grid-shapedelectrodes via said semiconductor optical waveguide in order to injectcarriers into said semiconductor optical waveguide.
 12. The opticalpacket header identifier according to claim 11, wherein the encoding ofsaid plurality of sets of tilted gratings is determined depending onwhether or not current is conducted to said grid-shaped electrodesprovided along an optical axis.
 13. The optical packet header identifieraccording to claim 12, wherein said optical beam focusing meanscomprises a “fø” lens provided such that a central axis of said core ofsaid optical waveguide and a light receiving face of said first photoreceiver have relationship represented by “fø”, and wherein a tilt angleof each of said plurality of sets of tilted gratings is determined suchthat diffracted optical beams diffracted by each of said plurality ofBets of tilted gratings are focused onto said first photo receiver bysaid “fø” lens.
 14. The optical packet header identifier according toclaim 12, wherein light-diffracting efficiency of each of said pluralityof sets of tilted gratings is determined such that intensity ofdiffracted optical beams from all of said plurality of sets of tiltedgratings becomes uniform by controlling current supplied to each of saidplurality of grid-shaped electrodes.
 15. The optical packet headeridentifier according to claim 11, wherein said optical beam focusingmeans comprises a slab waveguide provided to guide said diffractedoptical beams and having a parbolic reflecting end face for reflectingsaid guided optical beams, and wherein said first photo receiver isprovided at a focal point of said slab waveguide.
 16. The optical packetheader identifier according to claim 11, wherein said optical beamfocusing means comprises a second optical fiber for diffracting saiddiffracted optical beams to cladding mode by using tilted gratingsformed in a core portion of said second optical fiber, and wherein saidfirst photo receiver is provided at an end face of said second opticalfiber to receive said optical beams traveling in cladding mode withinsaid second optical fiber.
 17. The optical packet header identifieraccording to claim 16, wherein said optical beam focusing means furthercomprises a third optical fiber located between said optical waveguideand said second optical fiber, for focusing said diffracted opticalbeams from said optical waveguide onto a center of said second opticalfiber.
 18. The optical packet header identifier according to claim 17,wherein said optical waveguide is provided on a front side of said thirdoptical fiber, and positioned apart a distance two times a focal lengthof said third optical fiber for focusing, from said third optical fiber,and wherein said second optical fiber is provided on a rear side of saidthird optical fiber and positioned apart a distance two times a focallength of said third optical fiber from said third optical fiber. 19.The optical packet header identifier according to claim 17, furthercomprising a fourth optical fiber, wherein a center of said opticalwaveguide is positioned at a front focal point of said third opticalfiber, and wherein a center of said second optical fiber is positionedat a rear focal point of said fourth optical fiber, and wherein saidthird and fourth optical fibers focus diffracted optical beams from saidoptical waveguide onto said second optical fiber.